1. Field of the Invention
This invention relates to a machine tool which carries out machining based on relative movement between a work and a tool as the relative position and the relative angle between the work and the tool are changed in a plane including at least a work's non-circular cross-section. The invention also relates to a method for machining, a program and an NC data generation device.
2. Description of Related Art
In these days, in the field of machining of mechanical parts or metal materials, a 5-axis controlled machine tool, as shown in Patent Document 1, is in use. This 5-axis controlled machine tool has three linear axes, namely an X axis, a Y axis and a Z axis, extending perpendicular to one another, and two rotational axes about two linear axes selected from the above mentioned three linear axes. The three linear axes and the two rotational axes may be controlled simultaneously. Such 5-axis controlled machine tool may roughly be classified into two types, namely a 5-axis controlled machining center comprised of a conventional machining center added by two rotational axes, and a 5-axis controlled complex machining device. This 5-axis controlled complex machining device is an NC lathe on which a main spindle capable of a milling operation is mounted for performing a swinging movement. In each of these two machine tool types, a turning operation and a milling operation may be performed on a single machine tool. In addition, in the 5-axis controlled machine tool, even if a work is of a non-circular cross-section and complex in shape, such as a turbine blade or a fan blade for an aircraft jet engine, may be machined as the tool is tilted with respect to the work to change the tool position (tool orientation). Such turbine blade or fan blade is referred to below as a blade.
In view of complex movements performed by the above mentioned 5-axis controlled machine tool, the machining program for the 5-axis controlled machine tool is generated in general by a computer aided manufacturing device, referred to below as a CAM device. Such CAM device includes a tool foremost point control function which is in play at the time of concurrent 5-axis machining in which the linear axes and the rotational axes of the machine tool are driven simultaneously. Specifically, the tool foremost point control function operates not only to change the tool orientation but also correct for the tool length with changes in tool orientation in order to exercise control to cause the foremost part (tip) of the tool member to travel along a command tool path at a command speed. In giving a driving command at the time of concurrent 5-axis machining by such tool foremost point control, the tool orientation is crucial.
For example, in machining e.g., a blade with its complex shape, it is necessary to continuously change the tool orientation in order to prevent the tool member from conflicting against portions of the work other than the work points being cut or against component members of the 5-axis controlled machine tool. Moreover, if, in case of machining with a tool member, such as a bail end mill, an axial line of the tool member, referred to below as a tool axis, is coincident with a normal line drawn to the work surface, the cutting speed may not be increased at the foremost part of the tool. As a consequence, machining carried out may not be optimum. Thus, to cut a work with a tool member, such as a ball end mill, it is necessary to get the tool member tilted with respect to the line normal to the work surface in order to provide for improved surface properties of the finished product. That is, in giving a command by tool foremost point control in concurrent 5-axis machining, tool orientation control becomes crucial, and hence a method for setting the tool orientation becomes crucial.
In a CAM software ‘ESPRIT’ by DP TECHNOLOGY, four methods are shown as the methods for setting the tool orientation in a 5-axis controlled machine tool. One of these is a ‘surface square or angular’ method, in which the tool member is set at right angles to the work surface, with the tool axis coinciding with a line normal to the work surface, or at an angle inclined a preset angle with respect to the line normal to the work surface. The second one is ‘a point traversing’ method, in which the tool axis is aligned with a straight line interconnecting a specified point and the foremost part of the tool member, with the tool member being directed to the specified point. The third one is a ‘fixed vector’ method in which the tool member is oriented so that the tool axis is at a fixed angle irrespectively of the shape of the surface being machined. The last or fourth method is a ‘profiling’ method, in which the tool member is oriented so that the tool axis is aligned with a straight line interconnecting a specified curve and the foremost part of the tool member over a shortest distance.
However, if the tool orientation is set using the above mentioned four setting methods, the following problems may arise in manufacturing a blade with a non-circular cross-section by machining. For example, if the tool orientation is set using the ‘surface square or angular’ method, there is produced an area where the tool orientation is rapidly varied in machining the work with a non-circular cross-section because the tool member is set at right angles to or at an angle with respect to the work surface. In such area, the tool member needs to be moved at a speed (or acceleration) several times as high as that in the remaining area. It is not possible to drive the tool member at such driving speed, depending on the driving capability of a driving unit of the 5-axis controlled machine tool that drives the tool member. There is thus a risk that the cutting speed becomes lower than that in the remaining area such that it is not possible to maintain a constant cutting feed rate.
More specifically, the case of producing a blade with an elliptical cross-section 100 by machining will be explained with reference to FIG. 16. In this case, the orientation of a tool member 110 is rapidly changed along the long axis 120 in a first area 100a (an area from Q1 to Q2 in FIG. 16) and in a second area 100b (an area from Q3 to Q4 in FIG. 16) than in a remaining area 100c on the tool path along which the foremost part of the tool member 110 moves. That is, referring to FIGS. 17(A) to (C) and 17(E) to (G), the driving unit of the 5-axis controlled machine tool is able to drive the blade 100 in rotation in the C-direction, as well as to drive the tool member 110 along the feed direction F, in the remaining area 100c. The tool member 110 may be set at a surface square position on the surface of the blade 100 for cutting at a command feed rate for cutting.
However, if the driving unit of the 5-axis controlled machine tool attempts to set the tool member 110 in the first area 100a at a surface square position with respect to the surface of the blade 100, at a command feed rate for cutting, as shown in FIGS. 17(C) to 17(E), the driving unit is unable to drive the tool member because the orientation of the tool member 110 is varied rapidly in the first area. Hence, the cutting feed rate is lower than that in the remaining area 100c, with the result that a constant cutting feed rate may not be realized.
In similar trimmer, the orientation of the tool member 110 is rapidly varied in the second area 100b as well, and hence the driving unit of the 5-axis controlled machine tool is unable to drive the tool member 110 at the command cutting feed rate. That is, the cutting feed rate is lower than that in the remaining area 100c, with the result that the cutting feed rate may not be maintained constant.
In addition, if the method for tool orientation other than the ‘surface square or angular’ method is used to set the tool orientation, there is similarly produced an area where the tool is driven at a speed (or acceleration) several times faster than that in the remaining area. There is thus a risk that the cutting feed rate is lower in such area, with the result that it is not possible to maintain a constant cutting feed rate.
Furthermore, as shown in FIG. 16, in preparing the blade 100 by machining, continuous machining is not possible unless the orientation of the tool member 110 at a machining start point Qs on the tool path is coincident with that at a machining end point Qe at which the machining comes to an end after revolution of the blade 100 through 360°. However, if the method for tool orientation other than the ‘surface square or angular’ method is used, there is fear that the tool orientation of the tool member 110 at the start point Qs differs from that at the end point Qe after revolution of the blade 100 through 360°. As a result, the blade 100 may not be prepared by continuous machining.
There is thus a demand for a tool orientation setting method different from the above mentioned four methods. With this tool orientation setting method, different from the above mentioned four methods, it is necessary to provide for uniform changes in the tool orientation and for a constant angular velocity of tool tilting attendant on changes in tool orientation, in order that no load exceeding the driving capability of the 5-axis controlled machine tool will be imposed on the 5-axis controlled machine tool. In addition, the cutting feed rate from the start point Qs up to the end point Qe of machining on the tool path needs to be kept constant. Furthermore, the orientation of the tool member 110 needs to be the same at the end point Qe as at the start point Qs even after revolution of the blade 100 through 360°.